This invention relates to an adhesion enhancing coating and coated paper for heat and pressure fused toner particles, and more particularly to a coating and coated paper for use on security or business documents produced using noncontact printing devices such as laser or xerographic printers which provides improved adhesion of the toner particles to the printed security or business document produced.
Business forms, labels, bar codes, and security documents are printed on a wide variety of commercial printing devices. Traditional mechanical impact printers have been used in the past for many of these applications, especially in the imprinting of information on security documents such as checks. The mechanical impact produced by the printers, whether based on formed characters or dot matrix, generally provides information which adheres quite well to the underlying paper document. However, such mechanical impact printers have limited speed, high noise levels, and high costs for parts and maintenance.
With the advance of microcomputer technology, a number of faster printing methods have been developed to take advantage of the high-speed printing output which is now possible. Recently developed nonimpact printers are fast, quiet, and potentially more reliable because of fewer moving parts. Laser printers are one class of these newer nonimpact printers. Such printers operate by turning on and off a computer-controlled laser beam of light in a specific pattern onto an image cylinder or drum to form a latent image of positive and negative static charges.
As the image cylinder is rotated, toner particles from a toner cartridge are deposited on the image areas on the cylinder and held there by the static charges. As the image cylinder continues to rotate, the now visible toner image on the cylinder is then transferred to a paper web which has been statically charged to attract the toner particles and has the correct level of conductivity required. A corona wire positioned adjacent the image cylinder then erases the cylinder so that a new cycle may begin.
Typically, the toner image on the paper is then fused by passing the paper through a pair of rolls which apply both heat and pressure to the paper. This fusing by the application of heat and pressure is designed to bond the toner particles permanently to the paper. Descriptions of noncontact printers such as laser printers, the toners used therein, and the papers used for printing on them are known. See, for example, "New Papers for New Printers", Chemtech (1986), the disclosure of which is hereby incorporated by reference.
While laser printing is fast and quiet, the process has had significant limitations which have prevented its wide use in printing certain types of documents such as checks, other security documents, labels, and documents having bar code information thereon. These limitations include the inability to achieve satisfactory toner bonding on a large variety of paper products used to make such documents. For example, documents such as checks which contain not only payee and amount information but also MICR coding for automated handling must be able to withstand multiple handling and sorting cycles in high speed automated machinery. If the toner containing information such as MICR or bar coding on the document flakes off or is otherwise removed during such operations, the document will be rejected from the system and will have to be handled manually.
Additionally, because of the lack of strong adherence of toner to paper, documents printed using laser printers are subject to deliberate alteration by counterfeiters, forgers, and the like. For example, check amounts and/or payee information may be readily scraped off and new amounts substituted by the unscrupulous. While it may be possible to adjust the heat and/or pressure fusing steps which adhere the toner particles to paper as the information is printed, care must be taken not to overheat or melt the toner particles or scorch the paper stock.
Still further, papers used in laser printing systems must have a very narrow range of volume and surface resistivities to insure that the toner image is properly transferred from the image cylinder or drum. Such papers are also required to have a relatively high degree of surface smoothness and flatness, a specific range of moisture contents, and resistance to curl. Fabrication of special papers to have these characteristics increases the costs of such papers and their use.
Attempts have been made previously in the art of xerography to improve toner adhesion to substrates. Some of these efforts have been directed to the modification of the toner particles themselves. For example, Mitsuhashi, U.S. Pat. No. 4,499,168, added both a vinyl-containing polymer as well as polyethylene to toner particles to improve image fixing by the fuser rolls in a xerographic process. Sawai et al, U.S. Pat. No. 4,254,201, taught the use of a pressure sensitive adhesive added to the toner particles which was exuded under heat and pressure by the fuser rolls to fix the toner to a substrate.
A number of prior art workers have used various coatings on substrates in an attempt to improve toner adhesion. For example, Kuehnle, U.S. Pat. No. 4,510,225, coated a layer of a thermoplastic polymer on a substrate which was then preheated so that the toner particles would become embedded in that layer. Van Dorn, U.S. Pat. No. 2,855,324, taught the use of resin-coated paper to improve toner transfer, while Insalaco, U.S. Pat. No. 3,130,064, taught dipping a record card in a toluene solution containing a styrene-n-butyl acrylate copolymer in an attempt to improve toner adhesion.
A problem which is encountered with the use of coatings on papers is that the coatings are subjected to heat and pressure in the fuser rolls and may delaminate from the paper onto the hot fuser rolls, fouling the rolls or other parts of the printer into which they come into contact. Moreover, if the particular coating changes the surface properties of the paper or alters the paper's handling characteristics, further problems may result in the feeding, handling, printing, and ejection of the paper from the printer. For example, for those coatings which are applied from a solution which penetrates the paper surface, handling characteristics of the resulting coated paper may be altered.
Accordingly, there remains a need in the art for a paper product which provides enhanced toner adhesion for noncontact printed products without the drawbacks of prior art products.